System, apparatus, and method for in-vivo assessment of relative position of an implant

ABSTRACT

A method for providing an in-vivo assessment of relative movement of an implant that is positioned in a living being is provided that comprises a first assembly and a second assembly that are positioned within the living being. The first assembly comprises a passive electrical resonant circuit that is configured to be selectively electromagnetically coupled to an ex-vivo source of RF energy and, in response to the electromagnetic coupling, generates an output signal characterized by a frequency that is dependent upon a distance between the first assembly and the second assembly at the time of the electromagnetic coupling.

This application is a continuation application of U.S. patentapplication Ser. No. 12/416,904, filed Sep. 27, 2011, which is acontinuation-in-part of U.S. patent application Ser. No. 11/613,645,filed on Dec. 20, 2006, which is a continuation of U.S. patentapplication Ser. No. 11/105,294, filed on Apr. 13, 2005, now U.S. Pat.No. 7,245,117, which claims priority to U.S. Provisional Application No.60/623,959, filed on Nov. 1, 2004. This application is also acontinuation-in-part of U.S. patent application Ser. No. 11/717,967,filed on Mar. 14, 2007, now U.S. Pat. No. 7,466,120, which is acontinuation-in-part of U.S. patent application Ser. No. 11/276,571,filed on Mar. 6, 2006, now U.S. Pat. No. 7,498,799 which is acontinuation-in-part of U.S. patent application Ser. No. 11/105,294,filed on Apr. 13, 2005, now U.S. Pat. No. 7,245,117, which claimspriority to U.S. Provisional Application No. 60/623,959, filed on Nov.1, 2004. U.S. patent application Ser. No. 11/717,967 also claimspriority to U.S. Provisional Application No. 60/782,313, filed on Mar.14, 2006. Further, this application is a continuation-in-part of U.S.patent application Ser. No. 12/175,803, filed on Jul. 18, 2008, which isa divisional of U.S. patent application Ser. No. 11/472,905, filed onJun. 22, 2006, which is a divisional of abandoned U.S. patentapplication Ser. No. 10/943,772, filed on Sep. 16, 2004, which claimspriority to U.S. Provisional Application No. 60/503,745, filed on Sep.16, 2003. Additionally, this application is a continuation-in-part ofU.S. patent application Ser. No. 11/157,375, filed on Jun. 21, 2005.This application also claims priority to U.S. Provisional ApplicationNo. 61/072,715, filed on Apr. 1, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to systems, apparatus andmethods for determining a relative position of at least a portion of animplant within a living being, and more particularly, to systems andapparatus for determining at least one positional variable of theimplant of choice.

2. Background Art

Implants, such as prosthetic implants, are often subject to forces overtheir implant life times that can cause at least a portion of theimplant to move relative to the desired implant position. Often, theundesired movement of the implant can cause undue wear on the implant,potential failure of the implant, and limitations on the mobility of thepatient. This undesired implant movement ultimately can cause thepatient to undergo a revision or replacement implant surgery, which, inaddition to the normal surgical risks, may not necessarily allow for thedegree of mobility that the original implant afforded due to scar tissueformation and other surgical limitations that could be inherent to theparticular implantation site.

The system, apparatus and methods of embodiments described hereinovercome at least the above-described disadvantages by providing anability to accurately and non-invasively monitor the relative positionof an implant positioned within the living being. As one willappreciate, it is desirable to gather information relating to therelative position of the implant in order to monitor the structuralviability of the implant; to characterize the mechanical behavior ofimplant materials and structures under loading conditions; and to detectearly sign of catastrophic failure events. Further, it is desirable tomonitor the precise relative position of the implant within the livingbeing non-invasively.

Exemplary biomedical applications of the relative position analysisdescribed herein, without limitation, include its use in acutemonitoring applications, such as, for example and without limitation,total disk replacement procedures and total knee replacement proceduresand chronic monitoring applications.

For example, a total disk replacement procedure is typically recommendedwhen the native disk is not able to perform its function as a cushionbetween two vertebrae. The total disk replacement procedure replaces thenative disk and with a disk implant that is configured to restoreappropriate spacing between adjoining vertebrae. Referring to FIG. 1,typically, a conventional artificial disk implant comprises end portions6 that are configured to contact the bone and, in some case, provide atemporary or permanent anchoring of the artificial disk implant onto oneor both vertebrae 1, 2 that under or overlie the native disk.Conventionally, the anchoring of the disk implant is achieved viamechanical fasteners, such as screws 11, biocompatible cements, and thelike. Conventionally, the disk implant also has pliant section 8 that isinterposed between the end portions and that is configured to emulatethe function of the original disk. The pliant section should, at leastin theory, be able to absorb shocks and allow motion to occur betweenthe two adjoining vertebrae while maintaining desired alignment. Thedegrees of freedom as well as the amplitude of the tolerated motion aredictated by design, choice of material, etc. It is beneficial to thesurgeon performing the disk replacement surgery to know the precisedistance measurements between various portions of the implant prosthesisand/or between portions of the implant prosthesis and the adjoiningtissues, such as, for example, bone tissue, both during the replacementsurgery and in the days, months, and years after the implantation of theimplant prosthesis.

SUMMARY

This application relates to an apparatus and methods for providing anin-vivo assessment of relative movement of an implant that is positionedin a living being. In one aspect, the apparatus comprises a firstassembly and a second assembly that are positioned within the livingbeing. In one aspect, the first assembly comprises a passive electricalresonant circuit that is configured to be selectivelyelectromagnetically coupled to an ex-vivo source of radio frequency(“RF”) energy and, in response to the electromagnetic coupling, togenerate an output resonant frequency that is dependent upon a distancebetween the first assembly and the second assembly at the time of theelectromagnetic coupling.

In one aspect, it is contemplated that the passive electrical resonantcircuit of the first assembly comprises an inductive-capacitance (“LC”)resonant circuit. It is also contemplated that the second assembly canbe optionally selected from a metallic element, a non-metallic elementthat is at least partially magnetic, and a passive electrical circuit.In a further aspect, if a passive electrical circuit is used in thesecond assembly, it is contemplated that a LC resonant circuit can beused.

In operation, at a first predetermined distance between the firstassembly and the second assembly, the first assembly can be configuredto generate a first output resonant frequency in response to theelectromagnetic coupling, and wherein, at a distance between the firstassembly and the second assembly that differs from the firstpredetermined distance, the first assembly will generate an outputresonant frequency in response to the electromagnetic coupling thatdiffers from the first output resonant frequency.

DETAILED DESCRIPTION OF THE FIGURES

These and other features of the preferred embodiments of the inventionwill become more apparent in the detailed description in which referenceis made to the appended drawings wherein:

FIG. 1 is a schematic showing an exemplary conventional implant disposedtherebetween adjoining vertebrae and secured with conventional pediclescrew fasteners.

FIG. 2 is a schematic showing a first assembly and a second assemblymounted thereon respective spaced potions of an implant, the first andsecond assemblies both comprising electrical resonant circuits.

FIG. 3 is a schematic showing a first assembly and a second assemblymounted thereon respective spaced potions of an implant, the firstassembly comprising an electrical resonant circuit and the secondassembly comprising a metallic or non-metallic element.

FIG. 4 is a schematic showing a plurality of first assemblies and aplurality of second assemblies mounted thereon respective spaced potionsof an implant, in one example showing a pair of opposed first and secondassemblies that both comprise electrical resonant circuits and a secondpair of opposed first and second assemblies in which the first assemblycomprising an electrical resonant circuit and the second assemblycomprising a metallic or non-metallic element.

FIG. 5 is a schematic showing the implant of FIG. 4 with portions of theimplant offset from the longitudinal axis of the implant.

FIG. 6 is a schematic showing a first assembly mounted therein a portionof the bone tissue of the vertebrae and a spaced second assembly mountedthereon a potion of an implant, the first assembly comprising anelectrical resonant circuit and the second assembly comprising ametallic or non-metallic element.

FIG. 7 is a schematic showing a first assembly mounted thereon a potionof an implant and a spaced second assembly mounted therein a portion ofthe bone tissue of the vertebrae, the first assembly comprising anelectrical resonant circuit and the second assembly comprising ametallic or non-metallic element.

FIG. 8 is a schematic showing a first assembly mounted therein a portionof a bore within the bone tissue of the vertebrae, the first assemblybeing positioned below and spaced from an exemplary fastener which canserve as a second assembly, the first assembly comprising an electricalresonant circuit and the second assembly comprising a metallic ornon-metallic element.

FIG. 9 schematically illustrates a typical screen-shot of a conventionalnetwork analyzer displaying the level of reflected power measured at theterminals of a single coil antenna connected to a S-parameter box.

FIG. 10 schematically illustrates a typical screen-shot of aconventional network analyzer displaying the level of reflected powermeasured at the terminals of a single coil antenna connected to aS-parameter box.

FIG. 11 schematically illustrates a coil inductor of the LC resonantcircuit having a longitudinal axis, showing the respective windings ofthe coil inductor spiraling about and extending along the longitudinalaxis, and further showing at least a portion of each winding of the coilis non-planer with respect to the longitudinal axis.

FIG. 12 schematically illustrates an exemplary substantially planar LCresonant circuit.

FIG. 13 is an exemplary cross-sectional perspective view of the LCresonant circuit of FIG. 12.

FIG. 14 schematically illustrates a coil inductor of an exemplary LCresonant circuit having a longitudinal axis,

FIG. 15 illustrates an exemplary interrogation system for communicatingwith the first assembly that is positioned within a body.

FIG. 16 is an exemplary block diagram of an exemplary coupling loopassembly for communication with a wireless sensor assembly.

FIG. 17A illustrates a exemplary coupling loop that is un-tuned and FIG.17B illustrates its equivalent circuit.

FIG. 18A illustrates a loop that is tuned and FIG. 18B illustrates itsequivalent circuit.

FIG. 19A illustrates a loop terminated into a receiver with a high inputimpedance and FIG. 19B illustrates its equivalent circuit.

FIG. 20 is a graph that illustrates the comparison of the frequencyresponse for tuned loops and the frequency response for un-tuned loopswith high input impedances at the receiver.

FIG. 21 schematically illustrated two stagger tuned loops.

FIG. 22 illustrates the assembly of two stagger-tuned loops 1002, 1004for transmitting the energizing signal to the passive electricalresonant circuit of the assembly and one un-tuned loop 1006 forreceiving the output signal.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, drawing, and claims, and theirprevious and following description. However, before the present devices,systems, and/or methods are disclosed and described, it is to beunderstood that this invention is not limited to the specific devices,systems, and/or methods disclosed unless otherwise specified, as suchcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular aspects only andis not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

As used throughout, the singular forms “a,” “an” and “the” includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to “an assembly” can include two or more suchassemblies unless the context indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

Commonly assigned U.S. patent application Ser. Nos. 12/175,803,11/717,967, 11/613,645, 11/472,905, 11/276,571, 11/157,375, 11/105,294,and 10/943,772 are incorporated herein by reference in their entirety.

Embodiments provided herein comprise an apparatus and method of usingsame that can be configured to provide an in-vivo assessment of relativemovement of an implant that is positioned in a living being. The implantcan be any implant that is introduced into the living being and, in onenon-limiting example, can comprise a prosthetic device for preservingmotion between adjacent bones. In another non-limiting example, theimplant can comprise an intervertebral cage.

Referring generally to FIGS. 1-8, in one aspect, the apparatus cancomprise a first assembly 10 and a second assembly 20 that areselectively positioned within the living being in a spaced relationship.In one aspect, the first assembly 10 can comprise a passive electricalresonant circuit 12 that can be configured to be selectivelyinterrogated with RF energy produced by a remote interrogator. Thetransmitted RF energy can be selected in order to selectivelyelectromagnetically couple the passive electrical resonant circuit. Asone will appreciate and as described in more detail below, the remoteinterrogator acts as an ex-vivo source of desired RF energy. In responseto the electromagnetic coupling, the first assembly 10 can be isconfigured to generate an output signal characterized by a frequencythat is dependent upon a distance between the first assembly 10 and thesecond assembly 20. In one aspect, at a first distance between the firstassembly and the second assembly, the first assembly is configured, inresponse to the electromagnetic coupling, to generate a first outputsignal having a first frequency, and wherein, at a second distancebetween the first assembly and the second assembly that differs from thefirst distance, the first assembly will generate an output signal havinga second frequency in response to the electromagnetic coupling thatdiffers from the first frequency. The change to the frequency, which canfor example be a change in the resonant frequency of the first assembly,allows for to the determination of the relative distance between therespective first and second assemblies.

Optionally, in various aspects, the second assembly 20 can comprise, forexample and without limitation, a metallic element 22, a non-metallicelement 24, and/or a passive electrical circuit 26. In one aspect, thenon-metallic element can comprise magnetic properties. In one aspect,the passive electrical circuit 26 of the second assembly can comprise apassive electrical resonant circuit. Optionally, in a further aspect,the passive electrical resonant circuit of the second assembly 20 can besubstantially identical to the passive electrical resonant circuit ofthe first assembly 10.

In one aspect, the passive electrical resonant circuit of the firstassembly can be an electro-mechanical transducer that is capable oftransforming a signal from one form of energy into another, namely frommechanical into electrical energy. In one aspect, it is contemplatedthat the passive electrical resonant circuit 12 of the first assembly 10can comprise an inductance-capacitance (“LC”) resonant circuit.Optionally, if used, the passive electrical resonant circuit 26 of thesecond assembly 20 can also comprise a LC resonant circuit. In thisaspect, it is contemplated that the resonate frequency of the LCresonant circuit of the second assembly 20 would differ from a resonatefrequency of the LC resonant circuit of the first assembly 10. Inanother aspect, the passive electrical resonant circuit 12 of the firstassembly 10, and optionally the second assembly 20, can comprise aself-resonant inductor circuit.

Conventionally, a passive (no battery) LC resonant circuit is composedof two electrical passive components that are connected in series: (a) acoil, or inductor (“L”), (b) a capacitor (“C”). Such a passiveelectrical circuit exhibits electrical resonance when subjected to analternating electromagnetic field. The electrical resonance isparticularly acute for a specific frequency value or range of theimpinging signal. When the impinging signal substantially reaches theresonant frequency of the LC resonant circuit inside the sensorassembly, a pronounced disturbance of the field can be detectedwirelessly. In the simplest approximation, the electrical resonanceoccurs for a frequency f, related to the value of L and C according toequation 1:

f=(2π(LC)^(1/2))⁻¹  (equation 1)

The passive electrical resonant circuit for the assemblies describedherein that utilize a passive electrical resonant circuit can befabricated, for example and without limitation, via MicroElectro-Mechanical Systems (“MEMS”) approach to sensor design, whichlends itself to the fabrication of small sensors that can be formedusing biocompatible polymers as substrate materials. In a furtheraspect, appropriately biocompatible coatings can be applied to thesurfaces of the respective assemblies in order to prevent adhesion ofbiological substances to the respective assemblies that could interferewith their proper function. In one example, the passive electricalresonant circuit of the assembly can be manufactured usingMicro-machining techniques that were developed for the integratedcircuit industry. An example of this type of sensor features aninductive-capacitive resonant circuit with a variable capacitor isdescribed in Allen et al., U.S. Pat. No. 6,111,520, which isincorporated herein by reference. In this sensor, the capacitance varieswith the pressure of the environment in which the capacitor is placed.Consequently, the resonant frequency of the exemplary LC circuit of theAllen pressure sensor varies depending on the pressure of theenvironment.

The proximity of magnetic materials, such as, for example and withoutlimitation, ferrite or permanent magnet, can be detected wirelessly bymonitoring the resonant frequency of the exemplary wireless LC resonantcircuit. When the magnetic material of the second assembly remains farenough away from the LC resonant circuit of the first assembly, theresonant frequency of the first assembly circuit can be calculated usingequation 1 above. As the LC resonant circuit of the first assembly isbrought into proximity of the magnetic material of the second assembly,the resonant frequency of the LC resonant circuit of the first assemblychanges as a direct result of the change in the inductance value of theLC resonant circuit. In one aspect, magnetic materials with highrelative permeability at the frequency of interest can have aparticularly strong effect on the value of the inductance of the LCresonant circuit.

FIG. 9 schematically illustrates a typical screen-shot of a conventionalnetwork analyzer displaying the level of reflected power measured at theterminals of a single coil antenna connected to a S-parameter box. Thedip in the curve indicates the resonance of the LC resonant circuit ofthe first assembly, in the frequency domain, in absence of magneticmaterials. The dip was experimentally observed to shift to the left,i.e., decrease, as the second assembly, here an exemplary piece ofmagnetic material, e.g., ferrite, was brought in close proximity to theLC resonant circuit of the first assembly. The embodiments of the systemdescribed herein below comprising the apparatus described above and awireless interrogation system can to monitor the resulting change infrequency. After calibration, the noted shift in frequency can beanalyzed to determine the proportional change in distance between the LCresonant circuit of the first assembly and the magnetic material of thesecond assembly.

Similarly, a frequency shift can be observed when an LC resonant circuitof the first assembly 10 is brought into close proximity to an LCresonant circuit of the second assembly 20. In this example, therespective LC resonant circuits of the first and second assemblies havefundamental resonant frequencies fo₁ and fo₂ that are predictablycalculated using equation 1. However, as the respective LC resonantcircuits are brought into proximity to each other, the resonantfrequency of both of the assemblies changes as a result of a change inthe mutual inductance. It is also contemplated that the Q factor canalso change as the respective LC resonant circuits are brought intoproximity to each other.

FIG. 10 schematically illustrates a typical screen-shot of aconventional network analyzer displaying the level of reflected powermeasured at the terminals of a single coil antenna connected to aS-parameter box. The two respective dips in the curve indicate thefundamental resonance of the LC resonant circuits of the respectivefirst and second assemblies, in the frequency domain. The respective dipof the LC resonant circuit of the first assembly was experimentallyobserved to shift to the left, i.e., decrease, and the respective dip ofthe LC resonant circuit of the second assembly was experimentallyobserved to shift to the right, i.e., increase, as the LC resonantcircuit of the second assembly was brought in close proximity to the LCresonant circuit of the first assembly. The embodiments of the systemdescribed herein below comprising the apparatus described above and awireless interrogation system is able to monitor the resulting change infrequency. After calibration, the noted shift in frequency of the firstassembly can be analyzed to determine the proportional change indistance between the LC resonant circuit of the first assembly and theLC resonant circuit of the second assembly. Of course, in thisembodiment, it is also contemplated that the shift in frequency of thesecond assembly can be analyzed to determine the proportional change indistance between the LC resonant circuit of the first assembly and theLC resonant circuit of the second assembly.

As described above, it is contemplated that the LC resonant circuit cancomprise a coil inductor operably coupled to a capacitor. In variousaspects, the inductance of the LC resonant circuit can be between about0.1 to about 1000 micro-Henry, preferably between about 1 to about 100micro-Henry, and more preferably between about 5 to about 15micro-Henry. The capacitance of the LC resonant circuit can be betweenabout 0.1 to about 1000 pF, preferably between about 0.5 to about 100pF, and more preferably between about 1 to about 20 pF. The resonantfrequency of the LC resonant circuit can be between about 0.1 to about450 MHz, preferably between about 1 to about 60 MHz, and more preferablybetween about 25 to about 45 MHz. In addition, the quality factor atself resonance and the frequency range of the self-resonant frequencyitself can be between about 5 to 120, preferably between about 5 toabout 80, and more preferably between about 10 to about 70.

There are various manufacturing techniques that can be employed torealize sensors assemblies according to the current invention.Capacitors and inductors made by a variety of methods can bemanufactured separately, joined through interconnect methods andencapsulated in non-permeable packaging. In one embodiment, the pressuresensitive capacitor and the three-dimensional inductor coil are formedseparately and joined together to form the LC circuit. In anotherembodiment, the capacitor and inductor coil can be manufactured integralwith one another. Additionally, there are several methods to createthese discrete elements and to join each discrete element to create thefinal sensor assembly.

Q factor (Q) is the ratio of energy stored versus energy dissipated. Thereason Q is important is that the ring down rate of the sensor assemblyis directly related to the Q. If the Q is too small, the ring down rateoccurs over a substantially shorter time interval. This necessitatesfaster sampling intervals, making sensor detection more difficult. Also,as the Q of the sensor increases, so does the amount of energy returnedto external electronics. Thus, in one aspect, the sensor assembly can beconfigured with values of Q sufficiently high enough to avoidunnecessary increases in complexity in communicating with the sensorassembly via external electronics.

The Q of the sensor assembly can be dependent on multiple factors suchas, for example and without limitation, the shape, size, diameter,number of turns, spacing between the turns and cross-sectional area ofthe inductor component. In addition Q will be affected by the materialsused to construct the sensor assembly. In one example, the sensorassembly can be formed from materials with low loss tangents to effect asensor assembly with higher Q factors.

In one aspect, the coil inductor of the LC resonant circuit can be asubstantially planar spiral inductor. Optionally, the coil inductor ofthe LC resonant circuit can have a longitudinal axis and the respectivewindings of the coil inductor can spiral about and extend along thelongitudinal axis. In this aspect and referring to FIG. 11, at least aportion of each winding of the coil is non-planer with respect to thelongitudinal axis. For example, in a representative cross-sectionalplane that is substantially transverse to the longitudinal axis,portions of the windings in the y-axis can be below the cross-sectionalplane and portions of the winding in the y-axis can be above thecross-sectional plane.

In one aspect, the inductor coil can be comprised of the inductor coilbody and the coil leads. One skilled in the art will appreciate thatnumerous parameters of the inductor coil can be varied to optimize thebalance of size and the electrical properties of the circuit, includingthe materials, coil diameter, wire gage, number of coil windings, andcross-sectional area of the coil body. Typically, the material of thecoil must be highly conductive and also biocompatible. Suitablematerials include, but are not limited to, gold, copper and alloysthereof. If the wire is sufficiently strong, the coil can beself-supporting, also known as an “air core” configuration. A solenoidcoil is another suitable configuration. If the wire is not sufficientlystrong to be unsupported to maintain its intended configuration duringassembly and in use, the coil can be formed around a central bobbincomprised of a suitable dielectric material. In the alternative, thewound coil can be encased in a liquid polymer that can cure or otherwiseharden after it is applied to the coil body. Polyimide is one preferredmaterial for this application because of its thermal, electrical, andmechanical properties. However, processes achieving substantiallysimilar results that involve lower processing temperatures would makeother polymer choices desirable, such choices being obvious to oneskilled in the art.

Optionally, it is contemplated that the passive electrical circuit ofthe sensor assembly can be housed within a substantially non-permeableenclosure to ensure the protection of the passive electrical circuit ofthe sensor assembly when the respective sensor assembly is positionedwithin the living being. In this aspect, the passive electrical circuitof the sensor assembly can be protected from deleterious agents such ascorrosion, parasitic excessive strain/stress, biological response, etc .. . As one will appreciate, it is contemplated that the enclosure can beformed of materials that substantially prevent any undesired fluidsand/or gases from passing or diffusing through the walls of theenclosure, utilizing manufacturing processes that eliminate undesiredholes that could otherwise permit such passing of undesired fluids orgases. In another aspect, the enclosure can be formed of materials thatdo not allow any undesired fluids and/or gases from passing or diffusingthrough the walls of the enclosure. Exemplary enclosure material caninclude, without limitation, biocompatible polymer (such as, for exampleand without limitation, PEAK, PE, PTFE, FEP, semi-crystallinethermoplastic polymers, and the like), glass, fused-silica, lowtemperature glass, ceramics, quartz, pyrex, sapphire, sintered zirconiaand the like. An acceptable level of permeability can be a rate of fluidingress or egress that changes the original capacitance of the LCcircuit by an amount preferably less than 10 percent, more preferablyless than 5 percent, and most preferably less than 1 percent over theaccumulated time over which measurements will be taken.

Optionally, it is also contemplated that the housing can define aninternal cavity in which at least a portion of the passive electricalcircuitry can be disposed. In a further aspect, a known and invariantquantity of gas can be added to the internal cavity of the housing. Inanother aspect, it is contemplated that the enclosure can be formed ofmaterials that will not allow the resonant circuit of the assembly toflex in response to relative motion of the implant that the sensorassembly is mounted thereon or other forces that can be otherwiseapplied to the exterior surface of the assembly.

In another aspect, the exemplary enclosure materials help to provide thedesired biocompatibility, non-permeability and/or manufacturingprocessing capabilities of the assembly containing the resonant circuit.These exemplary materials are considered dielectrics, that is, they arepoor conductors of electricity but are efficient supporters ofelectrostatic or electroquasistatic fields. A dielectric material hasthe ability to support such fields while dissipating minimal energy. Inthis aspect, the lower the dielectric loss, the lower the proportion ofenergy lost, and the more effective the dielectric material is inmaintaining high Q.

With regard to operation within the human body, there is a secondimportant issue related to Q, namely that blood and body fluids areconductive mediums and are thus particularly lossy. As a consequence,when an assembly having a resonant circuit is immersed in a conductivefluid, energy from the sensor assembly will dissipate, substantiallylowering the Q and reducing the assembly-to-electronics distance. In oneaspect, the loss can be minimized by further separation of the assemblyhaving the resonant circuit from the conductive liquid, which can beaccomplished, for example and without limitation, by coating at least aportion of the assembly having the resonant circuit in a suitablelow-loss-tangent dielectric material.

In various aspects, it is contemplated that at least one of the firstassembly or the second assembly can be operably coupled to a portion ofthe implant. It is also contemplated that at least one of the firstassembly or the second assembly can be operably coupled to bone tissueproximate the implant. Optionally, both of the respective first andsecond assemblies can be operably coupled to spaced portions of theimplant. In another aspect, it is contemplated that both of therespective first and second assemblies can be operably coupled toadjacent bone tissue.

Further, one skilled in the art will appreciate that it is contemplatedthat a portion of the implant itself can form the second assembly. Inthis aspect, the portion of the implant that forms the second assemblycan be formed of magnetic materials, such as, for example and not meantto be limiting, a magnetized metallic material.

It is further contemplated that the first assembly can comprise aplurality of first assemblies and the second assembly can comprise aplurality of second assemblies. As one will appreciate, by using aplurality of opposed first and second assemblies, the respectivedistances between respective pairs of the opposed first and secondassemblies can be determined and the relative position of implant can bederived from the determined distances when compared to the known shapeof the implant and the known original implanted distances between therespective pairs of the opposed first and second assemblies.

It is contemplated in various aspects that the respective first andsecond assemblies can be positioned as described above and asexemplified in the figures to sense, in non-limiting examples, movementof at least a portion of the implant relative to at least anotherportion of the implant, movement of at least a portion of the implantrelative to at least a portion of the tissue of the patient, such as,for example and without limitation, adjacent bone tissue, and the like.

In another example, it is contemplated to fixate a first assembly at thebase of a bore hole in a bone tissue of the patient. The first assemblycan exemplarily be fixed using a conventional bone cement. Subsequently,a fastener, such as, for example and without limitation, a pediclescrew, can be mounted therein the bore hole such that it is positionedat a distance from the mounted first assembly. In this example, thesecond assembly can be mounted to or can be integral with the fastener.In one aspect, the fastener itself can serve as the second assembly. Itwill be appreciated that the apparatus and system described herein canbe used in this aspect to monitor the relative position of the fastenerto the first assembly. In operation, a shift in the sensed resonancefrequency of the first assembly, i.e., a sensed increase in theresonance frequency of the first assembly, can be an indication ofmovement of the fastener outwardly away from the first assembly.Optionally, it is of course contemplated that a second assembly can bemounted therein the bore hole and the first assembly can be mounted toor formed integrally within the fastener.

As described above, in one embodiment, an assembly having a resonantcircuit can comprise a passive LC resonant circuit with a varyingcapacitor. Because the exemplary assembly can be fabricated usingpassive electrical components and has no active circuitry, it does notrequire on-board power sources such as batteries, nor does it requireleads to connect to external circuitry or power sources. These featurescreate a assembly which is self-contained within the enclosure and lacksphysical interconnections that traverse the hermetic enclosure orhousing.

Because of the presence of the inductor in the LC resonant circuitsdescribed herein, it is possible to couple to the assembly having the LCresonant circuit electromagnetically and to induce a current in the LCresonant circuit via a magnetic loop. This characteristic allows forwireless exchange of electromagnetic energy with the assembly and theability to operate it without the need for an on-board energy sourcesuch as a battery. Thus, using the system described herein, it ispossible to determine the relative movement between the respective firstand second assemblies by a simple, non-invasive procedure by remotelyinterrogating the assembly or assemblies, detecting and recording theresonant frequency, and converting this value to a strain or stressmeasurement.

In a further aspect, the system for providing an in-vivo assessment ofrelative movement of an implant in a living being of embodimentsdescribed herein can comprise an ex-vivo source of RF energy and therespective first and second assemblies described above. In one aspect,the first assembly can comprise passive electrical resonant circuitpositioned within the living being that is configured to be selectivelyelectromagnetically coupled to the ex-vivo source of RF energy. Thefirst assembly can be configured to generate an output signalcharacterized by a frequency that is dependent upon a distance betweenthe first assembly and the second assembly in response to theelectromagnetic coupling.

In a further aspect, the system can comprises a means for monitoring theoutput signal of the first assembly, which frequency can comprises theresonant frequency of the first assembly. In one exemplary aspect, themeans for monitoring the output signal of the first assembly cancomprise a means for detecting or otherwise receiving the output signalof the first assembly and a processor, or similar processing means,configured to determine the relative distance between the respectivefirst and second assemblies based on the frequency of the output signalproduced by the first assembly. It is of course contemplated that, ifthe second assembly is an LC resonant circuit, the system can optionallycomprise a means for monitoring the resonant frequency of the outputsignal from the second assembly to determine the relative distancebetween the respective first and second assemblies based on thefrequency of the output signal of the second assembly.

In another aspect, the system described herein provides for a systemcapable of determining the resonant frequency and bandwidth of the firstassembly using an impedance approach. In this approach, an excitationsignal can be transmitted using a transmitting antenna toelectromagnetically couple an assembly having a passive electricalresonant circuit to the transmitting antenna, which resultingly modifiesthe impedance of the transmitting antenna. The measured change inimpedance of the transmitting antenna allows for the determination ofthe resonant frequency and bandwidth of the passive electrical resonantcircuit of the assembly.

In a further aspect, the system described herein provides for a transmitand receive system configured to determine the resonant frequency andbandwidth of a resonant circuit within a particular assembly. In thisexemplary process, an excitation signal of white noise or predeterminedmultiple frequencies can be transmitted from a transmitting antenna andthe passive electrical resonant circuit of the assembly iselectromagnetically coupled to the transmitting antenna. A current isinduced in the passive electrical resonant circuit of the assembly as itabsorbs energy from the transmitted excitation signal, which results inthe oscillation of the passive electrical circuit at its resonantfrequency. A receiving antenna, which can also be electromagneticallycoupled to the transmitting antenna, receives the excitation signalminus the energy which was absorbed by the assembly. Thus, the power ofthe received or output signal experiences a dip or notch at the resonantfrequency of the assembly. The resonant frequency and bandwidth can bedetermined from this notch in the power. In one aspect, the transmit andreceive methodology of determining the resonant frequency and bandwidthof a passive electrical resonant circuit of an assembly can includetransmitting a multiple frequency signal from a transmitting antenna toelectromagnetically couple the passive electrical resonant circuit onthe sensor assembly to the transmitting antenna in order to induce acurrent in the passive electrical resonant circuit of the assembly. Amodified transmitted signal due to the induction of current in thepassive electrical circuit is received and processed to determine theresonant frequency and bandwidth.

In another aspect, the system can determine the resonant frequency andbandwidth of a passive electrical resonant circuit within a particularassembly by using a chirp interrogation system, which provides for atransmitting antenna that is electromagnetically coupled to the resonantcircuit of the assembly. In this aspect, an excitation signal of whitenoise or predetermined multiple frequencies can be applied to thetransmitting antenna for a predetermined period of time to induce acurrent in the passive electrical resonant circuit of the assembly atthe resonant frequency. The system then listens or otherwise receives anoutput signal that radiates from the energized passive electricalresonant circuit of the assembly. In this aspect, the resonant frequencyand bandwidth of the passive electrical resonant circuit can bedetermined from the output signal. In this aspect, the chirpinterrogation method can include transmitting a multi-frequency signalpulse from a transmitting antenna; electromagnetically coupling apassive electrical resonant circuit on an assembly to the transmittingantenna to induce a current in the resonant circuit; listening for andreceiving an output signal radiated from the energized passiveelectrical signal of the assembly; determining the resonant frequencyand bandwidth from the output signal, and resultingly, determining thedistance between the respective first and second assemblies from thedetermined resonant frequency and bandwidth.

In a further aspect, the system described herein can provide an analogsystem and method for determining the resonant frequency of a passiveelectrical resonant circuit within a particular assembly. The analogsystem can comprise a transmitting antenna coupled as part of a tankcircuit, which, in turn, is coupled to an oscillator. In this aspect, asignal is generated which oscillates at a frequency determined by theelectrical characteristics of the tank circuit. The frequency of thissignal is further modified by the electromagnetic coupling of thepassive electrical resonant circuit of the assembly. This signal can beapplied to a frequency discriminator that provides a signal from whichthe resonant frequency of the resonant circuit can be determined. Inthis aspect, the analog method can include generating a transmissionsignal using a tank circuit that includes a transmitting antenna;modifying the frequency of the transmission signal byelectromagnetically coupling the passive electrical resonant circuit ofthe assembly to the transmitting antenna; and converting the modifiedtransmission signal into a standard signal for further application.

One exemplary method of interrogation is explained in more detail incommonly assigned U.S. patent application Ser. No. 11/105,294. In thedescribed methodology, the interrogating system energizes the assemblyhaving the resonant circuit with a low duty cycle, gated burst of RFenergy having a predetermined frequency or set of frequencies and apredetermined amplitude. The energizing signal is coupled to the passiveelectrical resonant circuit via a magnetic loop. The energizing signalinduces a current in the passive electrical resonant circuit that ismaximized when the frequency of the energizing signal is substantiallythe same as the resonant frequency of the passive electrical resonantcircuit. The system receives the ring down response of the assembly viamagnetic coupling and determines the resonant frequency of the assembly,which is then used to determine the relative distance between therespective first and second assemblies. In one aspect, the resonantfrequency of the assembly is determined by adjusting the frequency ofthe energizing signal until the phase of the ring down signal and thephase of a reference signal are equal or at a constant offset. In thismanner, the energizing signal frequency is locked to the assembly'sresonant frequency and the resonant frequency of the assembly is known.The relative distance can then be ascertained.

In one aspect, the system can comprise a coupling loop that can beselectively positioned relative to the first and second sensorassemblies to maximize the electromagnetic coupling between the passiveelectrical resonant circuit of the assembly and the coupling loop. Thesystem can also provide the necessary isolation between the energizingsignal and the output signal. In one aspect, it is contemplated that thesystem can energize the passive electrical resonant circuit of theassembly with a low duty cycle, gated burst of RF energy having apredetermined frequency or set of frequencies and a predeterminedamplitude. The energizing signal can be electromagnetically coupled tothe passive electrical resonant circuit of the assembly via one or moreenergizing loops. In operation, each energizing loop can be tuned to adifferent resonant frequency. The selection of the desired resonantfrequencies can be based on the desired bandwidth, which, in one aspectof the invention and without limitation can range between about 30 toabout 37.5 MHz.

The energizing signal induces a current in the passive electricalresonant circuit of the assembly that is maximized when the energizingfrequency is the same as the resonant frequency of the passiveelectrical resonant circuit of the assembly. The system receives thering down response of the assembly (or assemblies) via one or morecoupling loops and determines the resonant frequency of the sensor,which can be used to calculate the distance between the respective firstand second assemblies.

In one aspect, a pair of phase locked loops (“PLLs”) can be used toadjust the phase and the frequency of the energizing signal until itsfrequency locks to the resonant frequency of the passive electricalresonant circuit of the assembly. In one embodiment, one PLL samplesduring the calibration cycle and the other PLL samples during themeasurement cycle. In one non-limiting example, these cycles canalternate every 10 microseconds and can be synchronized with the pulserepetition period. In one aspect, the calibration cycle adjusts thephase of the energizing signal to a fixed reference phase to compensatefor any system delay or varying environmental conditions. Theenvironmental conditions that can affect the accuracy of the reading caninclude, but are not limited to, proximity of reflecting or magneticallyabsorbative objects, variation of reflecting objects located withintransmission distance, variation of temperature or humidity which canchange parameters of internal components, and aging of internalcomponents.

In one aspect, one of the PLLs can be used to adjust the phase of theenergizing signal and is referred to herein as the fast PLL. The otherPLL can be used to adjust the frequency of the energizing signal and isreferred to herein as the slow PLL. During the time that the energizingsignal is active, a portion of the signal enters the receiver and isreferred to herein as a calibration signal. The calibration signal isprocessed and sampled to determine the phase difference between itsphase and the phase of a local oscillator. The cycle in which thecalibration signal is sampled is referred to as the calibration cycle.In one aspect, the system can adjust the phase of the energizing signalto drive the phase difference to zero or another select reference phase.

During the measurement cycle, the signal coupled from the passiveelectrical resonant circuit of the assembly (referred to herein as theoutput signal) can be processed and sampled to determine the phasedifference between the output signal and the energizing signal. Thesystem can then adjust the frequency of the energizing signal to drivethe phase difference to zero or other reference phase. Once the slow PLLis locked, the frequency of the energizing signal is deemed to match theresonant frequency of the passive electrical resonant circuit of theassembly. The operation of the slow PLL is qualified based on signalstrength so that the slow PLL does not lock unless the strength of theoutput signal meets a predetermined signal strength threshold.

In one aspect, a single un-tuned coupling loop can be is used. In thisexemplary aspect, the loop can be connected to an input impedance thatis high relative to the loop inductance. Optionally, multiple couplingloops can be used and each loop is tuned to a different resonantfrequency.

In another aspect, the loops can be connected to a base unit thatgenerates the energizing signal and processes the output signal via acable assembly. In this aspect, the cable assembly provides isolationbetween the energizing signal and the sensor signal by maximizing thedistance between the coaxial cables that carry the signals andmaintaining the relative positions of the coaxial cables throughout thecable assembly. In another exemplary aspect, the coaxial cables can bepositioned on opposite sides of an internal cable, approximately 180degrees apart. Shielding can also be used to isolate the energizingsignal from the output signal. In one aspect, it is contemplated thatadditional shielding can be provided around each of the respectivecoaxial cables.

In one aspect, FIG. 15 illustrates an exemplary interrogation system forcommunicating with the wireless apparatus described above that ispositioned within a body. Without limitation, it is contemplated thatthe system can be used in at least two environments: the operating roomduring implant and the physician's office during follow-up examinations.

In one exemplary embodiment, the interrogation system can comprise acoupling loop 100, a base unit 102, a display device 104, and an inputdevice 106, such as, for example and without limitation, a keyboard. Inone exemplary embodiment, the base unit can include an RF amplifier, areceiver, and signal processing circuitry. In one aspect, the couplingloop 100 can be configured to charge the passive electrical resonantcircuit of the assembly and then couple signals from the energizedpassive electrical resonant circuit of the assembly into the receiver.Schematic details of the exemplary circuitry are illustrated in FIG. 15.

The display 104 and the input device 106 can be used in connection withthe user interface for the system. In the embodiment illustrated in FIG.15, the display device and the input device are conventionally connectedto the base unit. In this embodiment, the base unit can also providesconventional computing functions. In other embodiments, the base unitcan be connected to a conventional computer, such as a laptop, via acommunications link, such as an RS-232 link. If a separate computer isused, then the display device and the input devices associated with thecomputer can be used to provide the user interface. In one embodiment,LABVIEW software can be used to provide the user interface, as well asto provide graphics, store and organize data and perform calculationsfor calibration and normalization. The user interface can record anddisplay patient data and guide a user through surgical and follow-upprocedures. In another aspect, an optional printer 108 can be operablyconnected to the base unit and can be used to print out patient data orother types of information. As will be apparent to those skilled in theart in light of this disclosure, other configurations of the system, aswell as additional or fewer components can be utilized with embodimentsof the invention.

In one embodiment, the coupling loop can be formed from a band ofcopper. In this aspect, it is contemplated that the coupling loopcomprises switching and filtering circuitry that is enclosed within ashielded box. The loop can be configured to charge the passiveelectrical resonant circuit of the assembly and then couple signals fromthe energized passive electrical resonant circuit of the assembly sensorinto a receiver. It is contemplated that the antenna can be shielded toattenuate in-band noise and electromagnetic emissions.

In an alternative embodiment for a coupling loop, as shown in FIG. 16,separate loops for energizing 702 and for receiving 704 are provided,although a single loop can be used for both functions. PIN diodeswitching inside the loop assembly can be used to provide isolationbetween the energizing phase and the receive phase by opening the RXpath pin diodes during the energizing period, and opening the energizingpath pin diodes during the coupling period. It is contemplated in thisembodiment that multiple energizing loops can be staggered tuned toachieve a wider bandwidth of matching between the transmit coils and thetransmit circuitry.

In one aspect, the coupling loop or antenna can provide isolationbetween the energizing signal and the output signal, supportsampling/reception of the output signal soon after the end of theenergizing signal, and minimize switching transients that can resultfrom switching between the energizing and the coupled mode. The couplingloop can also provide a relatively wide bandwidth, for example frombetween about X to about Y and preferable from between about 30 to about37.5 MHz.

In one embodiment, separate loops can be used for transmitting theenergizing signal to the passive electrical resonant circuit of theassembly and coupling the output signal from the energized passiveelectrical resonant circuit of the assembly. Two stagger-tuned loops canbe used to transmit the energizing signal and an un-tuned loop with ahigh input impedance at the receiver can be used to receive the outputsignal. The term “coupling loop” is used herein to refer to both theloop(s) used to receive the output signal from the energized passiveelectrical resonant circuit of the assembly (the “assembly couplingloop”), as well as the loop assembly that includes the loop(s) used totransmit the energizing signal to the passive electrical resonantcircuit of the assembly (the “energizing loop”) and the assemblycoupling loop(s).

During the measurement cycle, the assembly coupling loop can beconfigured to couple the output signal from the energized passiveelectrical resonant circuit of the assembly, which is relatively weakand dissipates quickly. In one aspect, the voltage provided to thereceiver in the base unit depends upon the design of the assemblycoupling loop and in particular, the resonant frequency of the loop.

In a further aspect, it is contemplated that the coupling loop can beun-tuned or tuned. FIG. 17A illustrates a loop that is un-tuned and FIG.17B illustrates its equivalent circuit. The loop has an inductance, L₁,and is terminated into the receiver using a common input impedance,which can, for example and without limitation, be 50 ohms. The voltageat the receiver, V₁, is less than the open circuit voltage of the loop,i.e., the voltage that would be coupled by the loop if the loop was notterminated, V_(s), and can be calculated as shown below.

$\begin{matrix}{V_{1} = {V_{s}\frac{50}{50 + {j\; \omega \; L_{1}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Where L1 is the inductance of the loop and ω=2πf, with f=frequency inhertz.

To maximize the voltage at the receiver, it is contemplated that theloop can be tuned. FIG. 18A illustrates a loop that is tuned and FIG.18B illustrates its equivalent circuit. In this aspect, the loop has aninductance, L₁, and a capacitance, C₁. The capacitance, C₁, is selectedso that it cancels the inductance, L₁ at the resonant frequency, i.e.,the series resonant circuit, C₁-L₁, is 0 ohms at the resonant frequency.At the resonant frequency the voltage at the receiver, V₁, equals thevoltage coupled by the loop, V_(s). One disadvantage of this type ofloop is that it is optimized for a single frequency. If the loop is usedin an environment where the frequency of the output signal is changing,then the capacitance is either changed dynamically or set to acompromise value (e.g., the loop is tuned to a single frequency withinthe band of interest).

To minimize this issue, another embodiment illustrated in FIGS. 19A and19B uses an un-tuned loop with a high input impedance at the receiver.FIG. 19A illustrates a loop terminated into a receiver with a high inputimpedance and FIG. 19B illustrates its equivalent circuit. In thisaspect, the input impedance at the receiver is selected so that theenergy lost due to the loop impedance, L₁, is relatively insignificant.Using Zin as the input impedance at the receiver, the voltage at thereceiver, V₁, is calculated as shown below.

$\begin{matrix}{V_{1} = {V_{s}\frac{Zin}{{Zin} + {j\; \omega \; L_{1}}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

Since Zin is much larger than jωL₁, this can be approximated by thefollowing equation

$\begin{matrix}{{V_{1} = {V_{s}\frac{\infty}{\infty + {j\; \omega \; L_{1}}}}},{{{or}\mspace{14mu} V_{1}} = V_{s}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

As shown by the foregoing equation, the use of a relatively high inputimpedance at the input of the receiver negates L₁ for all frequencies.In one embodiment, a high impedance buffer can be inserted between theloop and an exemplary 50 ohm receiver circuit. In this embodiment, thehigh impedance buffer is on the order of 1 Mohm while the impedance ofthe loop is on the order of 200 ohms. In other embodiments, it iscontemplated that the input impedance is at least two times the loopimpedance.

In one aspect, the frequency response within the band of interest ismore monotonic if the assembly coupling loop uses a high input impedanceat the receiver, than if a tuned loop is used with a 50 ohm inputimpedance. FIG. 20 compares the frequency response for tuned loops andthe frequency response for un-tuned loops with high input impedances atthe receiver. The y-axis represents the difference in measured frequencybetween a calibration system using a network analyzer and the loop. Thex-axis represents the frequency of the L-C standard used in themeasurements. Linear interpolation was used between measurement points.Band 1 corresponds to a loop resonant at 32 MHz, Band 2 corresponds to aloop resonant at 35 MHz, Band 3 corresponds to a loop resonant at 38MHz, and Band 4 corresponds to a loop resonant at 41 MHz. Bands 1-4correspond to a prior art design that uses switched capacitors banks tovary the loop resonance to achieve the needed bandwidth. Bands 5 and 6correspond to un-tuned loops.

Bands 1-4 illustrate a slope variation within the band of interest,which can affect the accuracy of measurements made using the loop. Bands5 and 6 illustrate that the variation within the band of interest isless than in the systems using a tuned loop. The more monotonicfrequency response of an un-tuned loop with a high input impedancerequires a simpler set of calibration coefficients to be used for thefrequency conversion calculation.

An alternative embodiment to using an un-tuned loop and a high inputimpedance is to use stagger-tuned loops. If stagger tuned loops are usedto receive the output signal, then the loops can be tuned in a mannersimilar to that described in the following paragraphs in connection withthe transmission of an energizing signal.

During the energizing mode, the energizing loop produces a magneticfield. The intensity of the magnetic field produced by the energizingloop depends, in part, on the magnitude of the current within the loop.In one aspect, the current is maximized at the energizing frequency ifthe impedance of the loop is essentially 0 ohms at the energizingfrequency. The resonant frequency of the loop is related to the loopinductance and capacitance, as shown below.

$\begin{matrix}{f_{o} = \frac{1}{2\pi \sqrt{L*C\; 1}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

The impedance of the loop is preferably 0 ohms over the frequency rangeof interest, which, in an exemplary operating environment, can be,without limitation between about 30 MHz to about 37.5 MHz. To achievethe desired impedance over the desired frequency range, two or moreloops can be stagger tuned as exemplarily shown in FIG. 21.

The resonant frequencies for the loops are based on the bandwidth ofinterest. If there are two loops, then the loops can be spacedgeometrically. In one exemplary non-limiting aspect, the resonantfrequency of the first loop is can be about 31 MHz and the resonantfrequency of the second loop can be about 36.3 MHz, which corresponds tothe pole locations of a second order Butterworth bandpass filter havingabout −3 dB points at about 30 MHz and about 37.5 MHz. Although FIG. 21illustrates two loops, it is contemplated that other embodiments can usea different number of loops, which provides coverage for a much widerfrequency range. In one aspect, the loops can be spaced logarithmicallyif there are more than two loops.

FIG. 22 illustrates the assembly of two stagger-tuned loops 1002, 1004for transmitting the energizing signal to the passive electricalresonant circuit of the assembly and one un-tuned loop 1006 forreceiving the output signal. In this aspect, the loops are parallel toone another with the un-tuned loop inside the stagger-tuned loops.Placing the loop used to receive the output signal inside of the loopsused to transmit the energizing signal helps to shield the output signalfrom environmental interferences. In one embodiment, the loops can bepositioned within a housing.

One will appreciate that the signal from an implanted passive assemblyis relatively weak and is attenuated by the surrounding tissue and thedistance between the assembly and the coupling loop. Optimizing theposition and angle of the coupling loop relative to the assembly canhelp maximize the coupling between the assembly and the coupling loop.In one aspect, the coupling loop can be positioned so that a planedefined by the assembly coupling loop is approximately parallel to theinductor within the passive electrical resonant circuit of the assemblyand the assembly is approximately centered within the sensor couplingloop. If the coupling loop is not positioned in this manner relative tothe inductor, then the strength of the output signal is reduced by thecosine of the angle between the sensor coupling loop and the inductor ofthe resonant circuit.

In yet another aspect, orientation features can be provided forpositioning the coupling loop relative to at least the first assembly tomaximize the coupling between the first assembly and the coupling loop.In one aspect, the orientation features can facilitate the placement ofthe respective assemblies during implantation and the placement of thecoupling loop during follow-up examinations. In one aspect, therespective assemblies and the coupling loop can include orientationfeatures that are visible using conventional medical imaging technology.In exemplary aspects, the orientation features on the assemblies caninclude radiopaque markings and the orientation features on the couplingloop can include a pattern in the ribbing of the housing for the loop.

In one exemplary aspect, to facilitate the proper coupling of thesystem, the assembly, the assembly housing, and/or the implant caninclude orientation features, which are visible using a medical imagingtechnology, such as fluoroscopy, to facilitate the placement of theassemblies during implantation and the coupling loop during follow-upexaminations. To position the coupling loop relative to the assembly,the coupling loop is moved or adjusted until a predetermined patternappears. In one aspect, the orientation features on the coupling loopcan be implemented as a pattern in the ribbing of the housing for theloop, which aids in positioning the coupling loop relative to theassembly of the implant. In one aspect, the housing includes anessentially circular section that can be smaller than the diameter ofsection. When assembled, the sensor coupling and energizing loops arepositioned within the ring-shaped section. The orientation features arelocated in the circular section.

To receive a output signal from the assembly, the physician positionsthe coupling loop so that the assembly having the passive electricalresonant circuit is positioned approximately at the center of thecoupling loop and the angle of the coupling loop is adjusted until thedesired orientation of the passive electrical resonant circuit of theassembly and the coupling loop is achieved, which places the inductorcoil within the passive electrical resonant circuit essentially parallelto the coupling loop. The orientation feature on the housing can aid inpositioning the coupling loop so that the sensor is at approximately thecenter of the loop.

In one aspect, isolation of the energizing signal and the output signalprovided by the base unit and the coupling loop can be maintained in thecable that connects the base unit to the coupling loop. In one aspect, acable can connect the base unit to the coupling loop and isolate theenergizing signal from the output signal. In one aspect, the distal endof the cable that connects to the base unit can comprise a multi-pinconnector (e.g., AL06F15-ACS provided by Amphenol) and a right anglehousing. The proximal end of the cable that connects to the couplingloop can comprise a first connector, which can be a multi-pin connector(e.g., AMP 1-87631-0 provided by Amphenol) that operably connects to thefiltering and switching circuitry associated with the loop; a secondconnector that operably connects to the energizing loop; and a thirdconnector that operably connects to the loop that couples the signalfrom the sensor. In this exemplary aspect, the right angle housing andthe strain relief provide strain relief at the respective ends of thecable. When assembled with the housing, the strain relief can bepositioned proximate to the housing. Optionally, other types of strainrelief can be implemented, including, without limitation, physicalconstraints, such as tie wraps, ferrals or epoxy, and/or service loops.In one aspect, the cable can also comprise ferrite beads, which can helpreduce ground currents within the cable.

In one aspect, the position of the coaxial cables within the cable isdesigned to maximize the isolation between the energizing signal and thesensor signal, while minimizing the diameter of the cable. The cable isconfigured to maximize the isolation between the coax cable thattransmits the energizing signal and the inner bundle and the twistedpairs and the coax cable that receives the sensor signal and the innerbundle.

Although several embodiments of the invention have been disclosed in theforegoing specification, it is understood by those skilled in the artthat many modifications and other embodiments of the invention will cometo mind to which the invention pertains, having the benefit of theteaching presented in the foregoing description and associated drawings.It is thus understood that the invention is not limited to the specificembodiments disclosed hereinabove, and that many modifications and otherembodiments are intended to be included within the scope of the appendedclaims. Moreover, although specific terms are employed herein, as wellas in the claims which follow, they are used only in a generic anddescriptive sense, and not for the purposes of limiting the describedinvention, nor the claims which follow.

1. A method for providing an in-vivo assessment of relative movement ofan implant in a living being, the method comprising: positioning a firstassembly comprising a passive electrical resonant circuit within theliving being; and positioning a second assembly within the living beingspaced from the first assembly; selectively electromagnetically couplingthe first assembly to an ex-vivo source of RF energy; and generating anoutput signal from the first assembly, in response to theelectromagnetic coupling, characterized by a frequency that is dependentupon a distance between the first assembly and the second assembly. 2.The method of claim 2, wherein the characterized frequency is theresonant frequency of the first assembly dependent on the distancebetween the first assembly and the second assembly.
 3. The method ofclaim 1, wherein the second assembly comprises a metallic element. 4.The method of claim 1, wherein the second assembly comprises anon-metallic element.
 5. The method of claim 4, wherein the non-metallicelement exhibits magnetic properties.
 6. The method of claim 1, whereinthe second assembly comprises a passive electrical circuit.
 7. Themethod of claim 6, wherein the passive electrical circuit of the secondassembly comprises a passive electrical resonant circuit.
 8. The methodof claim 7, wherein the passive electrical resonant circuit of thesecond assembly is substantially identical to the passive electricalresonant circuit of the first assembly.
 9. The method of claim 7,wherein the passive electrical resonant circuit of the second assemblycomprises a LC resonant circuit.
 10. The method of claim 9, wherein thepassive electrical resonant circuit of the first assembly comprises a LCresonant circuit.
 11. The method of claim 10, wherein a resonantfrequency of the second assembly differs from a resonant frequency ofthe first assembly.
 12. The method of claim 1, wherein the passiveelectrical resonant circuit of the first assembly comprises a LCresonant circuit.
 13. The method of claim 12, wherein the LC resonantcircuit of the first assembly comprises a coil inductor operably coupledto a capacitor.
 14. The method of claim 13, wherein the inductance ofthe LC resonant circuit is between about 5 to about 15 micro-Henry. 15.The method of claim 13, wherein the resonant frequency of the LCresonant circuit is between about 25 to about 45 MHz.
 16. The method ofclaim 13, wherein the capacitance of the LC resonant circuit is betweenabout 1 to about 20 pF.
 17. The method of claim 13, wherein, at a firstdistance between the first assembly and the second assembly, the firstassembly generates, in response to the electromagnetic coupling, a firstoutput signal having a first frequency, and wherein, at a seconddistance between the first assembly and the second assembly that differsfrom the first distance, the first assembly generates an output signalhaving a second frequency in response to the electromagnetic couplingthat differs from the first frequency.
 18. The method of claim 17,wherein the coil inductor is a substantially planar spiral inductor. 19.The method of claim 17, wherein the coil inductor has a longitudinalaxis and wherein the coil inducted is elongated about the longitudinalaxis.
 20. The method of claim 17, wherein at least a portion of eachwinding of the coil inductor is non-planar with respect to thelongitudinal axis.
 21. The method of claim 1, wherein at least one ofthe first assembly and the second assembly is coupled to a portion ofthe implant.
 22. The method of claim 21, wherein at least one of thefirst assembly and the second assembly is coupled to bone tissueproximate the implant.
 23. The method of claim 22, wherein both thefirst assembly and the second assembly are coupled to spaced portions ofthe implant.
 24. The method of claim 1, wherein a portion of the implantforms the second assembly.
 25. The method of claim 24, wherein theportion of the implant forming the second assembly is metallic.
 26. Themethod of claim 1, wherein the implant comprises a prosthetic device forpreserving motion between adjacent bones.
 27. The method of claim 1,wherein the implant comprises an intervertebral cage.